1. Field of the Invention
This invention relates generally to hard disk drives of the type generally used with microcomputers for storing digital data and more specifically to parking the heads in a disk drive at a controlled velocity.
2. Description of the Relevant Art
The problem addressed by this invention is encountered in disk drives used to store data in computers. FIG. 1 shows a block diagram of a typical disk drive system as is known in the prior art. More specifically, FIG. 1 shows a disk drive system comprised of a disk drive microprocessor 8, control logic 10, voltage fault detector 12, voice coil motor drivers 14, voice coil motor 16, read/write head electronics 18 head carriage 20, read/write heads 21, magnetic media 22, spindle motor 24, and spindle motor drivers 26. In operation, host computer 4 communicates through controller 6 to send and receive commands and/or data to the disk drive microprocessor 8. Responsive to these commands, the disk drive system rotates the spindle motor 24, and thus the magnetic media 22, at a substantially constant velocity. The voice coil motor 16 moves the heads 21 to specific tracks over the magnetic media 22. Once the heads have stabilized over the appropriate tracks, the heads can read data from the magnetic media or can write data on the media, as is known in the industry.
In a disk drive systems such as the one described in FIG. 1, the magnetic media 22 rotates at high velocities and the heads 21 fly at very close distances to the magnetic media 22. In this environment, designers are concerned about the head making contact with the media (a head crash) since such contact can have catastrophic results. Data can be permanently lost. Even worse, the heads or the media can be damaged such that the entire disk no longer functions. Therefore, virtually all modern disk drives design their systems to avoid as much head contact with the media as possible. To this end, many disk drives park their read/write heads when the disk drive is powered down so that the heads land on a parking zone instead of the area of the disk which has data. A parking zone is an area of the magnetic media where data is not stored which is typically the innermost tracks of the magnetic media. This minimizes the wear on the magnetic media where data is stored and thus increases the reliability of the disk drive and the integrity of the data.
FIG. 2 shows the schematic of a typical head parking circuit, as is known in the prior art. In this circuit, the voice coil motor 36 is controlled by the H-bridge formed by n-channel transistors 32, 34, 38, and 40, which collectively are the VCM drivers 14 of FIG. 1. When disk drive system is on, the gates of transistors 32, 34, 38, and 40 are connected control logic 10 as shown in FIG. 1 and are provided with the necessary signals to position the heads to a desired track. FIG. 2 shows the appropriate voltage signals to the VCM drivers for the VCM 36 to park the heads. More specifically, FIG. 2 shows a park voltage source 30 providing a park voltage to the A node of the VCM 36. The B node is connected to a voltage reference, ground, through transistor 40 and sense resistor 42. The park voltage can be supplied from the back electromotive force (BEMF) of the spindle motor as it spins down or from storage device such as a capacitor as is known in the art. A constant park voltage will accelerate the heads to a velocity where the BEMF generated by the movement of the heads plus the voltage drop due to the resistance of the VCM (times the current) is equal to the park voltage.
In this circuit, the park voltage is approximately equal to the voltage drop across the VCM 36 and the voltage drop across the sense resistor. The voltage drop across the VCM 36 is approximately equal to the voltage drop due to the back electromotive force (BEMF) due to the movement of VCM 36, plus the voltage drop due to the resistive losses in the VCM. Thus, assuming the voltage drop across transistor 40 is negligible, the equation for this relation can be simplified as: EQU V.sub.park voltage =V.sub.VCM BEMF +I(t)R.sub.VCM +I(t)R.sub.SENSE RESISTOR
where
V.sub.VCM BEMF =the voltage of VCM 36 due to the BEMF.
I(t)R.sub.VCM =the resistive voltage drop of VCM 36 as a function of current I(t).
I(t)R .sub.SENSE RESISTOR =the resistive voltage drop of the sense resistor as a function of the current I(t).
The equation also indirectly shows that V.sub.VCM BEMF is not precisely controlled. Since the current to achieve a given velocity is not precisely known, the BEMF which will result is not accurately defined by this circuit. Also, this circuit will asymptotically reach its target speed, but not very fast.
FIG. 3 shows a schematic diagram of a second head parking circuit as is known in the prior art. In this circuit, the voice coil motor 56 is controlled by the voice coil motor drivers 52 and 58, which collectively are the VCM drivers 14 of FIG. 1. When the disk drive system is on, the inputs of amplifiers 52 and 60 (through inverter amplifier 8) are connected to control logic 10 as shown in FIG. 1 and are provided with the necessary signals to position the heads to a desired track. FIG. 3 shows the schematic of the circuit when the power is turned off. A park voltage source 50 providing a park voltage to the input of amplifier 52 and to the input of inverting amplifier 60. The output of inverting amplifier 60 is connected to amplifier 58 so that amplifier 58 provides a current which is equal and opposite to the current provided by amplifier 52. Sense resistor 54 is connected in series to the VCM 56.
Assuming unity gain for amplifiers 52 and 58, the park voltage can be made to be approximately equal to the voltage drop across the VCM 56 and the voltage drop across the sense resistor 54. The voltage drop across the VCM 56 is approximately equal to the voltage drop due to the back electromotive force (BEMF) due to the movement of VCM 36, plus the voltage drop due to the resistive losses in the VCM. Thus, the equation for this relation can be simplified as: EQU V.sub.park voltage =V.sub.VCM BEMF +I(t)R.sub.VCM +I(t)R .sub.SENSE RESISTOR
where,
V.sub.VCM BEMF =the voltage of VCM 36 due to the BEMF.
I(t)R.sub.SENSE RESISTOR =the resistive voltage drop of VCM 36 as a function of current I(t).
I(t)R.sub.VCM =the resistive voltage drop of the sense resistor as a function of the current I(t).
Like the circuit in FIG. 2, the equation also indirectly shows that V.sub.VCM BEMF is not precisely controlled. Since the current to achieve a given velocity is not precisely known, the BEMF which will result is not accurately defined by this circuit. Also, this circuit will asymptotically reach its target speed, but not very fast. A circuit which controls the BEMF (i.e. the speed of the VCM) provides both a more accurate velocity and a faster settling time at the desired velocity.